http://m.windowscentral.com/windows-10-technical-preview-will-expire-april-15-2015I don't know where you read that April 15 date.
No one really knows the release schedule, but I'd have thought you can carry on using the preview on your test machine for as long as you like.
The Technical Preview that was released in October 2014 is the one that expires. We've already had another one released since then, so just keep up to date on it and all will be well.